This invention relates to a blend of a) fibrillated, organic, synthetic polymer, b) synthetic, organic polymer fiber staple and c) synthetic, organic, soluble polymer particles and its incorporation into wet mix non-asbestos type friction materials for the purpose of improving their processability, green strength and in many cases, improving the physical properties of the resultant cured friction material. Wet mix roll linings are often produced by shaping small modules of stock in wheel-rolling machine. The machine consists of two wheels rotating slowly in opposite directions. The upper wheel rotates as a close fit between flanges on the lower wheel and the distance between the flanges determines the width of the liner while the spacing between the wheels determines its thickness. Dough from a mixer is broken down into small granules and passed through the nip of the wheels. The dough is shaped and pressed in the proper density and the resultant continuous ribbon of dough is then cut into strips of the required length for processing into brake linings, disk pads, truck block, off highway brakes, clutch facings and the like. Solvent is used in the stock to dilute the resin binder in order to expedite the wetting of the dry components. After forming the linings, the bulk of the solvent is removed by drying. Any remaining solvent is removed during the early stages of the step-cure. As is well recognized, it has become incumbent upon the industry to find a cost-effective replacement for asbestos in friction materials because of the health, environmental and safety hazards attributed to asbestos. Numerous approaches to the replacement of asbestos have led to a substantial body of technology and prior art that has resulted in at least two major categories of non-asbestos formulations. They are: 1) semi-metallic materials, and 2) organic non-asbestos materials. These materials are more fully discussed in U.S. Pat. No. 4,866,107, hereby incorporated herein by reference.
The elimination of asbestos from friction material formulations, although relatively successful, has caused, however, various other problems not the least of which is difficult processing blends of ingredients for the manufacture of linings, the reduced strength and toughness of linings produced from other ingredients, the increased cost of said ingredients vis-a-vis asbestos and the physical and frictional performance of said finished products compared to asbestos-containing materials. Additionally, many of the asbestos replacement type formulations for friction materials have failed to achieve success due to reduced frictional/thermal stability properties of the molded friction material which render them less competitive.
Most attempts to eliminate asbestos fibers from friction material formulations have centered around the use of other organic and inorganic fibrous materials, alone or in conjunction with a myriad of different components.
For example, U.S. Pat. No. 4,145,223 incorporates glass fibers and ceramic fibers whereas U.K. Published Application No. 2027724A employs preoxidized acrylic fibers. Similarly, U.S. Pat. No. 4,197,223 and U.K. Patent No. 1604827 teach mixtures of inorganic and organic fibers such as glass fibers, mineral wools, alumina-silicate fibers, wood pulp, jute, sisal and cotton linters. Aramid fibers are taught in U.S. Pat. Nos. 4,374,211 and 4,384,640 and acrylic fibers are shown in U.S. Pat. Nos. 4,418,115; 4,508,855; 4,539,240 and 4,656,203; G.B. Published Application No. 2,129,006A and Japanese Published Application Nos. 87/106,133; 87/89,784 and 87/149,908.
Additionally, in U.S. Pat. No. 4,324,706 there is disclosed the combination of pulp-like particles of heat-resistant aromatic polymeric materials, inorganic or organic fibrous materials, friction-regulating agents and thermosetting polymer binders.
U.S. Pat. No. 4,866,107 claims a composition of a thermosetting binder resin, a fibrous reinforcing material and a fibrillated acrylonitrile polymer-based fiber of an Efficiency Index from about 0.8 to about 2.0.
European Published Patent Application No. 0,282,004 discloses a reinforcing mixture for friction products employing a polyacrylonitrile wet gel containing an additive comprising polyethylene glycol esters of pelargonic acid, enanthic acid, caprylic acid, capric acid and blends thereof etc.
Recently issued U.S. Pat. No. 5,106,887 teaches the formation of non-asbestos friction materials comprising fibrillated acrylic fibers admixed with glass fibers, heat resistant organic fibers, inorganic fibers or metallic fibers wherein the fibrillated acrylic fibers have a CANADIAN STANDARD FREENESS (CSF) of at least 450 ml whereas U.S. Pat. No. 5,004,497 claims a friction material comprising 0.85-30%, by weight, of carbon fibers and 2-20%, by weight, of aramid fibrillated and chopped fibers. The material may contain 3-20%, by weight, of polyimide dust, melamine dust, cashew dust or phenol dust. These dusts are cured, thermoset materials and not soluble and therefore do not fall within the scope of the present invention. The '887 patent does not mention the inclusion of organic, synthetic polymer particles and, in fact, specifically discloses that the organic fibers are aramid pulp, a fibrillated fiber. No organic, synthetic polymer particles are mentioned.
Moreover, PCT Published Application No. WO93/04300 teaches the production of a composite friction material comprising a matrix resin, a fiber reinforcing material and aramid particles. The fibrous reinforcing material may be pulp or floc, but not both.
All of the above cited references fail to recognize the unique cooperative effect which is achieved by employing the unique blend of the present invention. The references either fail to teach the use of particles of synthetic, soluble organic polymer or, if such particles are suggested, fail to include one or both of the other critical components of the present invention. More specifically, U.S. Pat. No. 4,324,706 teaches pulp-like particles such as fibers, films, flakes or ribbons each provided with a plurality of tentacle-like projections in combination with staple fibers. No polymer particles having diameters of less than 60 microns are disclosed in the '706 patent, the particles of this reference being more akin to fibrillated fiber component a) hereof than the particles c).
U.S. Pat. No. 4,866,107 teaches a blend of a fibrillated fiber and other organic, synthetic polymer fibers but does not mention that said other fibers are staple or that particles of organic, synthetic polymer must be employed therewith.
The WO93/04300 published application is probably the closest prior art as relates to the instant invention. The '300 application utilizes aramid particles as wear additives in the formation of friction materials in conjunction with fibers in the form of floc or pulp. The floc is described as fibers cut to lengths of 1-10 mm whereas the pulp is described as fibrillated fibers. Both the pulp or floc are preferably composed of aramid-type polymers. The aramid particles range from 10-250 microns in size, the smallest being described as providing processing assistance by aiding the opening of the fibrillated fiber during mixing but the application does not discuss wet mix green strength benefits. The instant friction materials differ from those taught by the '300 application in that herein there is employed both a fibrillated fiber and a fiber staple in conjunction with the soluble, organic, synthetic polymer particles. This combination of ingredients has been found to provide unexpectedly superior results with respect to green strength and in many cases, superior physical, frictional/thermal properties.
Related patents which show blends of fibrous materials and polymer particles include U.S. Pat. No. 3,325,345 which is limited to fibrillated cellulosic fibers; U.S. Pat. No. 4,387,178 which requires the presence of a polyacrylic latex; U.S. Pat. No. 4,485,138 which requires the presence of rubber to prepare a vulcanized blend of fibers; U.S. Pat. No. 4,495,030 which includes submicron size glass fiber in a toxic vapor absorptive filter material; U.S. Pat. No. 4,748,075 which teaches a soft gasketing material composed of at least three (3) different fibers, natural fibers, synthetic organic fibers and mineral or metal fibers. No organic, synthetic, soluble polymer particles are added thereto.
U.S. Pat. No. 4,769,274 teaches the production of inexpensive mats using a coarse, cellulosic fiber, thermoplastic synthetic polymer fibrils and non-fibrous, thermoplastic, synthetic polymer particles. The products are used as door panels, interior/exterior partitions, molded doors, etc., when laminated with other disclosed ingredients. No disclosure of friction materials is made.
U.S. Pat. No. 5,190,657 is related to blood filters comprised of specific denier interlocked, textile fibers and certain fibrillated particles of polymeric material as taught in U.S. Pat. No. 4,274,914. The particles are described as not being fibers.
U.S. Pat. No. 5,272,198, by the present inventors, relates to a reinforced material comprised of an elastomeric matrix and a small denier acrylic fiber which may be used in conjunction with other fibers such as glass fibers, polyolefin fibers, polyamide fibers, polyester fibers, polyimide fibers etc. No particles of synthetic, soluble, organic polymer are added.